And no, there are no error in the maillog except for the mentioned one and for the valid ones. (There are many "postfix/smtpd[18654]: NOQUEUE: reject: RCPT from XXX Recipient address rejected: User unknown in virtual mailbox table;" messages. But they are valid because those user accounts really do not exist.)

I updated now to 3.0.3.1 and I have still the same problems. (I have to say that I didn't expect anything else because I haven't seen any closed tickets that would have something to do with this issue.)

[root@shared postfix]# /usr/bin/maildrop -v
maildrop 2.5.1 Copyright 1998-2005 Double Precision, Inc.
GDBM/DB extensions enabled.
Maildir quota extension are now always enabled.
This program is distributed under the terms of the GNU General Public
License. See COPYING for additional information.